Fuel cells that generate electric power using a fuel gas (hydrogen-containing gas) and an oxygen-containing gas (air in general) have recently been developed for next-generation energy. A fuel cell (hereinafter may be simply referred to as cell) includes a solid electrolyte layer disposed between a fuel electrode layer and an oxygen electrode layer. The cell generates electric power by delivering a fuel gas to the fuel electrode layer and an oxygen-containing gas to the oxygen electrode layer, and heating the cell to a temperature of 1000° C. to 1050° C. (see, for example, PTL 1).
Unfortunately, if such a cell is operated for a long time, some of the materials deteriorate due to high operation temperature and consequently cause power generation efficiency to decrease. Accordingly, PTL 2 discloses a technique for suppressing decrease in power output from the cell after a long-time use by controlling the structure of the oxygen electrode layer so as to contain grains having specific surface areas of 1.5 m2/g to 9.0 m2/g and pores having pore sizes in the range of 30 nm to 100 nm, thereby suppressing increase in reaction loss in the oxygen electrode layer.